Over-voltage transients in the form of electro-static discharge, electromagnetic interference, lighting, or in other harmful forms can strike integrated circuit (IC) packages unexpectedly. Therefore, transient suppressing measures are often necessary to ensure normal functionality over the expected life span of the packaged circuits.
In electronic systems such as cell phones, laptop computers, handheld GPS systems, or digital cameras where space is severely limited, transient voltage suppressor (TVS) devices made of semiconductor are the only viable choice to protect the sensitive IC chips in the systems. To merge the TVS functionality into the chips to be protected is mostly impractical because fabrication processes designed for most integrated circuits do not lend themselves to good TVS performance. For this reason, stand-alone semiconductor TVS devices remain the choice in the industry.
In the TVS devices, p-n junctions and the associated depletion regions combined with the resistive elements are designed to absorb the damaging energy of the transient strikes. As the transients are often manifested as fast, high voltage pulses, TVS devices are configured to force the p-n junctions into breakdown and thus divert the energy through these junctions rather than through the protected circuits.
Known TVS devices are built based on diffused lateral p-n junction diodes in silicon chips comprised of epitaxial silicon on a heavily doped n-type substrate. The diodes are fabricated by implanting or diffusing p-type dopant through windows openings cut through a grown or deposited oxide layer over the silicon to form p-n junctions under the silicon surface. A p-n junction thus formed has two parts—a relatively planar portion at a fixed distance from the surface of the silicon, and a non-planar cylindrical portion, which surrounds the planar portion at the periphery extending to the silicon surface. These p-n junctions are responsible to by-pass potentially damaging energy from the protected circuits without themselves suffering permanent damage.